My primary concerns are (in order of priority - and I only speak for myself) (a) consistency across platforms (b) minimize (unrequired) performance overhead I would prefer an api which is consistent for both win32, posix or other os which only did as much as what the user (us) wanted for example ... module System.Directory.ByteString ... FilePath = ByteString getDirectoryContents :: FilePath -> IO [FilePath] which is the same for both win32 and posix and represents raw uninterpreted bytestrings in whatever encoding/(non-encoding) the os provides....implicitly it is for the user to know and understand what their getting (utf-16 in the case of windows, bytes in case of posix platforms) then this api can be re-exported with the decoding/encoding by System.Directory/System.IO which would export FilePath=String ie a two level api...